Overview of bathymetric data for lower pond.

Overview of bathymetric data for lower pond.

Sub-bottom profiling systems are employed to identify and characterize layers of sediment or rock under the seafloor.

These systems also can be helpful in locating hard objects buried beneath the seafloor, such as shipwrecks. Substructure has employed sub-bottom profiling technology to assist in many hydrographic surveys.

In sub-bottom profiling, a sound source directs a pulse toward the seafloor. Parts of this sound pulse reflect off of the seafloor, while other parts penetrate the seafloor. The portions of the sound pulse that penetrate the seafloor are both reflected and refracted as they pass into different layers of sediment. 

These signals return toward the surface, where they are detected by hydrophones towed by a surface vessel:

  • The time it takes for the reflected sound pulses to return to the surface vessel can be used to determine the thickness of the sub-bottom layers in the seafloor and how the layers are positioned (e.g., level or sloped). The reflected sound also gives some limited information about the composition of the various layers.

  • Refracted sound pulses, which follow a more complex path, provide additional information about the sub-bottom layers. Through analysis of the seismic refraction, a more-comprehensive understanding of the density of various sub-bottom layers can be developed. The variability in density shows the relative differences of the seafloor, with greater density showing harder materials.

Implementing a sub-bottom system inevitably involves a trade-off between penetration into the seafloor and resolution.

Sound sources that produce lower-frequency pulses achieve greater penetration though the seafloor but produce a lower-resolution picture; higher-frequency pulses achieve higher resolution but do not penetrate as deeply into the sub-bottom strata. Although the relationship between penetration and resolution is not linear, it is fairly constant. 

The key to obtaining “optimal” results from a sub-bottom survey is understanding these frequency differences and then matching the objective of the survey with the appropriate equipment.

See Magnetometers.